This invention relates generally to storage tank heaters and, more particularly, to fuel fired heaters for tanks for the storage of fluids such as asphalt (i.e. bituminous material) or hot water.
The fuel fired tank heaters of the indicated type in general use today comprise a burner which is similar to a domestic oil burner and is fitted to a flue containing a ceramic combustion chamber. The ceramic combustion chamber has a rather thick wall and serves to shield the outside of the flue from overheating the asphalt for the length of the ceramic chamber.
It is important from an efficiency standpoint to permit as much of the transfer tube to be heated to the near optimum temperature for heat transfer without overheating. Overheating of the heating surface will cause many undesirable effects on the product such as separation of mixtures and emulsions, distillation of product, coking, changes in viscosity and characteristics of the material in storage as well as deterioration of the heating surface itself, such as pitting and corrosion.
It is the general object of this invention to provide a novel combustion chamber construction for use in the tank heaters of the indicated type which is a substantial improvement over the ceramic combustion chambers of the prior art. To this end, the combustion chamber in accordance with the invention comprises a plurality of tubes arranged one within another and spaced apart radially for control of the dissipation of the heat transferring radially from the combustion products to the exterior of the heat transfer tube in contact with the contents of the tank. This design controls the rate of dissipation of heat primarily by radiation of heat from one tube to another, then to the exterior of the heat transfer tube. By varying the number of tubes, the air gap between tubes, the length of tubes, the concentricity of the tubes, the temperature profile along the length and around the circumference of the heat transfer tube can be designed to correspond to the heat being released by combustion to equalize and balance the heat density to eliminate hot spots and irregularities found in similar equipment of the prior art.
Another significant advantage of the tube within a tube design of the invention results from the fact that this design involves much less mass than comparable ceramic tube combustion chambers and the ceramic tubes may be made of metal, such as steel, which had a very low specific heat as compared with the ceramic material used in the prior art combustion chamber. Thus, the combustion chamber will come up to efficient operating temperature more rapidly and after a long period of operation, there will be a very minimal temperature override with the tank heater in accordance with the invention as compared to a considerable temperature override which is caused by the hot ceramic used in the prior art designs which ceramic continues to give off heat over an extended period after the burner has been turned off.
Another advantage of the design in accordance with the invention is that it is less costly to manufacture than a comparable ceramic combustion chamber design. Also, the design permits the use of a smaller size primary flue in the combustion chamber area since the tube within a tube assembly occupies less space than a comparable design, reducing the amount of liquid in the tank required to cover the heating system making available more material for use and less in dead storage. Accordingly, a smaller size fire box for the burner may be used, again resulting in a savings of cost and a more efficient consumption of fuel.
Another feature of the inventive design is that by constructing the tubes of metal the unit is not subject to damage in shipment as is the case with the prior art ceramic insulator design.
This invention also permits air to be introduced between one or more of the tubes either by the eductor effect of the combustion gases passing through the transfer tube or by positive means to further control the surface temperature of the heating surface in contact with the product.